The present invention is directed to cooling passage repairs, and more particularly, to a method of repairing diffusion cooling passages and an electrical discharge assembly for carrying out the method.
Because of extreme operating conditions, gas turbine engines are known to employ components having thermal barrier coatings and cooling passages to protect the component by reducing temperature exposure during use. Thermal barrier coatings are selected to provide resistance to spallation (coating loss) during thermal cycling of the engine as well as resistance to oxidizing and corrosive environments. During normal engine operation and after time, a thermal barrier coating will degrade in certain surface areas subjected to strenuous operating conditions. Once a thermal barrier coating's protective ability has been reduced below an acceptable level, removal and reapplication of the coating is required.
Cooling passages allow film cooling of gas turbine components. Film cooling effectiveness can be increased by using diffusion passages that have a conical portion and an enlarged opening at the surface of the component. The shaping of the passages to diffuse air before it enters the boundary layer of the component broadens the spread of air downstream of the passage and thus, increases cooling effectiveness. Although high quality diffusion passages provide superior performance, they are both costly and difficult to form.
In the case of blade repair, the blade may first be stripped of its original coating and then a replacement coating is applied to the blade prior to returning it to service. During this repair process, cooling passages present in the blade may be partially or completely filled with repair or replacement coating material. Such excess material can accumulate in each cooling passage. This phenomenon is known as “coatdown” and can restrict the flow capacity of the affected passages. Coatdown can diminish the cooling effectiveness of the film cooling thereby reducing the component's useful operating life. Thus, any cooling passages that are subject to coatdown are typically unacceptable for return to service and require reworking to remove the excess material before the blade can be put back into service.
Electrical discharge machining (EDM) is a known process for producing shaped passages or other openings in metals. It uses electrical current discharges to erode metal. For example, by pulsing a direct current between a positively charged work piece (anode) and an electrode (cathode), a spark discharge may be produced. The discharge occurs when the potential difference between the electrode and the workpiece, which both contact a dielectric fluid, is great enough to break down the dielectric fluid and produce an electrically conductive channel. Upon application of a voltage or potential, a current flow results with enough heat energy to melt and erode the work piece. This process has application in the machining of small, deep, odd-shaped passages that are cumbersome to produce by other means.
Typical EDM methods for producing or remanufacturing diffusion passages in engine components use a copper electrode manufactured in a three-dimensional shape by stamping and coining. The electrode consists of at least one small diameter elongated end that produces a cooling air metering section. The elongated end connects to a three-dimensional diffuser-shaped portion that produces a diffused area for the diffusion passage. The electrode produces a similar shaped passage, with allowance for electrode overburn and EDM electrode erosion. Unfortunately, known EDM methods are time consuming to operate and are a relatively expensive process compared to other processes such as laser drilling. Furthermore, the copper electrodes of an EDM are fragile and not reusable.